The present invention relates to a telephone switching system and, more particularly, to a telephone switching system comprising a plurality of switch units each including a bypass unit for permitting a selected class or group of subscribers to make use of the system following certain malfunctions or failures within the system.
Telephone switching systems in which telephone subscribers are able to communicate with each other or with subscribers at one or more remote locations are well known to those skilled in the art. These systems commonly employ processors (e.g., computers) at each location for controlling or orchestrating all call processing operations by which subscribers may be connected to each other. These operations include such well known operations as the scanning of subscriber data lines to detect off-hook conditions, supplying dial tones, selecting available digital receivers for receiving dialed information, determining and selecting available trunk group lines, and, based on the identities of the various parties, formulating appropriate address information for connecting "calling" subscribers with "called" subscribers. The systems as described above generally include redundant processors so that if one processor should malfunction or fail and be required to be taken off line the other processor can be put on line to replace the malfunctioning or failed unit.
While the abovedescribed arrangement is generally satisfactory, there is ordinarily no provision for servicing subscribers, especially subscribers having a special and critical need to use the system (e.g., high-level military or government personnel), in the event both processors should malfunction or fail and be required to be taken off line. To this end, it has been proposed to use a patchboard arrangement for providing a limited or reduced number of connections between selected subscribers and trunk group lines, for example, up to ten percent of the normal connections, in the event both processors in a system should malfunction or fail and be taken off line. However, this patchboard approach has been determined to be undersirable since the patchboard arrangement would have to be of a substantial physical size to accommodate even a small group of subscribers (e.g., 60 subscribers) and could be incorporated into the system without interfering with the normal operation of the system only after the processors have been taken off line. This latter operation would be tedious and time consuming and lead to substantial downtime of the system.